1. Field of the Invention
The present invention relates to fiber optic probes for spectrophotometric analyses of scattered light. More particularly, the present invention relates to fiber optic probes for Raman spectroscopy of gaseous samples. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background:
Spectrophotometric techniques based on emission, absorption or scattering processes are frequently used for qualitative and quantitative analyses. The development of lasers and optical fibers has allowed the placement of sensitive equipment in locations remote from industrial process environments, making spectrophotometry applicable to a wide range of processes. Applications include on-line monitoring of industrial process streams, monitoring the contents of storage tanks, detection of toxic or explosive substances, studying the migration of subsurface contaminants, and monitoring air and water quality. Where measurements must be made at many different locations, a number of probes may be connected to a single remotely-located measuring instrument by optical fibers, as in the on-line process control monitoring system disclosed in commonly-assigned U.S. Pat. No. 5,131,746. Remote monitoring is essential in harsh chemical and radioactive environments.
Detection of hydrogen is a problem in many industrial process environments. Measurements of hydrogen concentration in the offgas of many chemical processes are used to detect potentially-explosive concentrations so that deflagrations can be prevented. Presently available hydrogen sensors have short lifetimes in corrosive environments, for example, when exposed to acids in an offgas stream. Also, the presence and variabilities of the concentrations of other gases such as carbon dioxide and nitrogen dioxide cause problems in detection and quantification. The large thermal conductivities of these gas mixtures coupled with their varying concentrations preclude the use of thermal conductivity sensors. High concentrations of nitrogen dioxide interfere with or poison currently available electrochemical sensors. Analytical techniques such as gas chromatography and mass spectrometry are capable of detecting hydrogen, however, these techniques require complex equipment that may be difficult or impossible to maintain when placed in a severe process environment, and neither is suited for remote measurements. Remote monitoring using spectrophotometric techniques and an in-situ fiber optic probe would allow detection of hydrogen--and other gases--in severe process environments.
The composition of a sample can often be determined from its absorption spectrum, that is, the frequency or wavelength distribution of the light absorbed by the sample. A typical system for absorption spectroscopy includes a light source, an optical probe containing light-transmitting and light-receiving fibers, and a detector. Light from the source is directed to the sample by the transmitting fiber, passes through the sample, and is transmitted to the detector by the receiving fiber. Measurements taken from a suitable reference sample are compared to measurements taken from the test sample to help determine the concentrations of various constituents in the test sample.
Absorption spectroscopy is useful for detecting and characterizing those analytes that have useful spectra in the ultraviolet (UV), visible (VIS) or near-infrared (near-IR) range. However, many analytes of interest, including hydrogen (H.sub.2), oxygen (O.sub.2), nitrogen (N.sub.2), methane (CH.sub.4), carbon dioxide (CO.sub.2) and carbon monoxide (CO), do not have any UV-VIS absorption lines; the "fingerprint" regions of the absorption spectra are in the IR range. Because presently-available optical fibers severely attenuate IR wavelengths, absorption spectroscopy is not suitable for remote detection of such analytes.
Optical techniques based on other mechanisms such as fluorescence, luminescence and Raman scattering may be used for detection. Raman spectroscopy is a sensitive analytical technique based on the inelastic scattering of light (typically, monochromatic light frown a laser) by a molecule. The rotational or vibrational energy of a molecule is changed as it is excited to a different energy level by the incident light. The transition to a final energy level is accompanied by emission of radiation. Thus, in addition to elastically-scattered light having the same wavelength as the exciting light, the scattered light contains small amounts of light with different wavelengths. When expressed in terms of the delta wave number (the difference between the laser wavelength and the wavelengths of the scattered light), a Raman spectrum consists of a series of lines starting close to the laser line and corresponding to the possible vibrational or rotational quantum numbers of the sample molecules. The spectrum is characteristic of the scattering molecules, with the intensities of the lines depending on the concentrations of the scattering molecules in the sample.
Vibrational and rotational Raman spectra are typically in the visible or near-IR region, therefore, Raman spectra are less severely attenuated than IR absorption spectra by transmission over optical fibers. In addition, Raman spectroscopy is particularly useful for identifying the constituents of a sample since Raman spectra contain more spectral lines, and sharper lines than obtained with other types of spectroscopy.
A major problem in Raman spectroscopy and other light scattering measurements is the low signal-to-noise ratio, that is, the very low intensity of the Raman-scattered light compared to the intensity of the exciting light. Raman spectroscopy, like absorption spectroscopy, is carried out with a light source, an optical probe, and a detector. Some of the exciting light--and some elastically-scattered light--is reflected back to the light-receiving fibers by the interior surfaces of the probe. Sensitive detectors with high light gathering power and high stray light rejection are needed to isolate and measure the low intensity Raman signal. Such instruments are costly and delicate, and are not well suited for use in many industrial process environments. Furthermore, monochromatic light transmitted by an optical fiber excites the fiber molecules, causing Raman scattering within the fiber itself. This "self-scattering" or "silica scattering" generates an additional signal that interferes with the Raman signal collected from the sample.
These problems are particularly evident for small samples and gases. The smaller the sample chamber, the more exciting light is reflected towards the light-receiving fibers by the chamber walls, and the more difficult to detect the Raman signal. The fewer the number of sample molecules, the smaller the Raman signal. Increasing the sample pressure provides more scattering molecules and therefore an increased Raman signal, but does not significantly change the amount of non-Raman-scattered light reaching the detector.
To improve the signal-to-noise ratio, filters may be used to remove a narrow band of wavelengths centered on the wavelength of the laser line. Chemometric techniques are used to factor out background noise and identify the signal of interest. However, the intensity of a typical Raman signal is much lower than the intensity of the non-Raman-scattered light reaching the light-receiving fibers. Small samples, such as are desirable in process environments or when dealing with hazardous materials, produce Raman signals that arc indistinguishable from background levels even after filtering and data analysis.
Various techniques are available for increasing the amount of light collected by the light-receiving fibers of an optical probe. Thus, lenses may be provided to direct light onto the fibers. The ends of the fibers may be tapered for improved coupling efficiency, as described in commonly-assigned and recently filed patent application Ser. No. 08/056,390 (Fiber Optic Probe Having Fibers With Endfaces Formed For Improved Coupling Efficiency), the disclosure of which is incorporated herein by reference.
Some devices incorporate indicators that exhibit a change in an optical property, such as fluorescence emission, color, and so forth, in response to the sample. For example, Riccitelli, et al. (U.S. Pat. No. 5,124,129) coat the inside of a transparent endotracheal tube or connector with a pit-sensitive dye suspended in a predominantly hydrophilic polymer matrix. Wolfbeis (U.S. Pat. No. 4,857,472) adds a fluorescent indicator to an SO.sub.2 -permeable polymer and coats the resulting composition onto a solid substrate. When the indicator is brought into contact with a SO.sub.2 -containing medium, the extent to which fluorescence is quenched is a measure of the SO.sub.2 content of the medium. Klainer, et al. (U.S. Pat. No. 4,892,383) describe a reservoir fiber optic chemical sensor having a semipermeable membrane to admit the analyte of interest into the cell body.
Despite the variety of fiber optic probes that are available, there is no known optical probe that addresses the problems posed by Raman scattering measurements of small, gaseous samples: low signal intensity and low signal-to-noise ratio due to interference from exciting light. Such a probe would allow the use of Raman spectroscopy for remote monitoring in a wide range of process environments.